In circuit-switched telephony, also referred to by the acronym PSTN (public switched telephone network), subscribers of the public telephone network are connected to the switching systems of the public switching technology assigned to them or responsible for them. Using subscriber access concentrators or subscriber access networks, frequently just referred to as access networks, located upstream of the switching-system, the subscriber accesses can be combined, but the result of this however is still circuit-based access by the switched subscriber to the switching system. The combining of the subscriber accesses is generally implemented using Time Division multiplexing—frequently abbreviated to TDM—for example PCM (Pulse Code Modulation) or SDH (Synchronous Digital Hierarchy).
Within the context of the growth in services which are based on variable bit rates, the subscribers of the public telephone network increasingly have access to packet networks, namely access to the Internet via broadband transmission networks such as LANs (Local Area Networks), networks based on DSL (Digital Subscriber Line) technology or cable networks. For subscribers with access to a packet network the obvious course is to use the lines or networks necessary for packet-based access for telephone traffic as well, instead of maintaining separate narrowband lines/networks.
For PSTN subscribers who access a packet-based network there is subscriber network access equipment such as IADs (Integrated Access Devices) or cable modems which terminate the subscriber-side transmission network, for example LAN (Local Area Network), network with DSL (Digital Subscriber Line) accesses or cable network. On the subscriber side this type of network access equipment has analog or ISDN (Integrated Services Digital Network) accesses; On the packet network side telephony signaling and useful data with certain security protocols are transmitted.
As well as this there are packet-based terminals, telephones based on the Internet protocol (IP) for example, with signaling protocols specifically defined for use in the packet network, such as the H.323 Standard defined by the International Telecommunications Union (ITU) or the SIP (Session Initiation Protocol) in accordance with the IETF (Internet Engineering Task Force) Standard.
Connection setup and connection control of telecommunications applications via packet networks generally involves a switching center which executes functions such as access control, signaling, call control and billing. This switching center can for example include an appropriately equipped server in the IP network—frequently called a gatekeeper—or a PSTN (public switched telephone network) switching system equipped for these additional packet network-related functions. In addition local resources for control or supervision functions can be provided in the network access equipment.
With conventional circuit-switched PSTN/ISDN connections many of the service features are based on the fact that the user channel is switched in the switching center and the necessary call processing resources are provided there as needs dictate.
An arrangement for packet data transmission over a packet network IPnet is shown in FIG. 1. For the connection of subscribers A-Tln and B-Tln the useful data dat is transmitted directly between subscriber devices TNE transmission, to which the two subscribers involved A-Tln and B-Tln are connected. The signaling information sig is routed separately from the useful data via a switching center Vst. The figure shows a switching center Vst with a Call Control Core CCC with an upstream packet-based adaptation unit PAE. Since the useful data dat is not routed in the switching center Vst, conventional resources, i.e. those available in PSTN networks, provided in the switching center Vst can no longer be made available for specific service features. Therefore the call processing resources are relocated and provided in the packet network IPnet in resource servers with specific functionality (e.g. Interactive Voice Response servers for interactive voice input/output, Conference servers for telephone conferences) or in subscriber-related equipment (e.g. network access equipment, Media Gateways etc.).
If services are requested by the A-side or the B-side in a connection, this frequently leads to a redirection or to a modified routing of the useful data. The useful data stream between A- and B-subscriber is generally to be divided up in at least one direction and directed to the relevant resource servers.
FIGS. 2 and 3 show two examples of the redirection of the useful data stream dat for implementing telecommunication services within the framework of a packet network-based connection of subscribers. By contrast with FIG. 1 the subscribers involved are in the area of responsibility of different switching centers Vst-A and Vst-B, positioned in the vicinity of the subscribers in each case. The signaling data sig for the subscribers B1-Tln and B2-Tln (FIG. 2) or A-Tln and B2-Tln (FIG. 3) passes from the equipment assigned to the subscribers TNE via the local switching center Vst-A. The signaling data sig of subscribers A-Tln and Tln (FIG. 2) or B1-Tln and Tln (FIG. 3) is routed via the local switching center Vst-B. The switching centers Vst-A and Vst-B each feature packet adaptation equipment PAE and Call Control Core CCC. To implement the service features the switching centers Vst-A and Vst-B are each connected to a Resource Server RS-B and RS-A (FIG. 2) or RS-A and RS-B (FIG. 3). Signaling zasig (zasig: for interoffice signaling) between the switching centers Vst-A and Vst-B can for example be undertaken with the aid of an expanded ISUP (ISDN user part) protocol, the BICC (bearer independent call control) protocol or the T-SIP (telephone session initiation protocol) protocol for packet network applications.
In the example of FIG. 2 the useful data stream dat of a connection between the subscribers A-Tln and B1-Tln is rerouted for a connection between the subscribers A-Tln and B2-Tln. This is referred to as a call transfer, which is initiated to forward a subscriber's call for example. FIG. 3 shows a rerouting of the useful data stream dat, in which a connection between the subscriber A-Tln and B1-Tln is interrupted, so that subscriber A-Tln can consult with subscriber B2-Tln. The call of subscriber B1-Tln is applied to the Resource Server RS-A. Music or a recorded announcement may be played while subscriber Tln-B1 is connected to the Resource Server RS-A. After the connection between subscriber A-Tln and B2-Tln is ended the original connection is restored.
For the call processing service features shown in FIGS. 2 and 3 the switching center Vst-B must be informed by interoffice signaling zasig about the changed routing and change its packet address assignment in a stable call state, even a number of times if necessary. Handling local, subscriber-related service features leads to additional signaling load in the interoffice signaling zasig, i.e. the signaling between the switching centers assigned to the subscriber and where necessary further intermediate switching centers and to additional adaptation in the remote switching centers. The necessary adaptation, such as changing the packet network addresses, must be supported by the remote switching centers, which is a very restrictive assumption for the network and its interoffice signaling or conversely determines the availability of the relevant service features depending on the relevant parties to the call. Even if the switching centers sup port the concurrent updating of the packet network addresses of their useful data stream, negative effects on the service quality can still arise from delays in the packet network and long delay times in interoffice signaling.